One of the important plant defense mechanisms that has evolved during the co-evolution of the plant-pathogen interaction is the so-called gene-for-gene interaction model (Flor, 1971). According to a simple formation of this model, plant resistance (R) genes encode specific receptors for molecular signals generated by avirulence (Avr) genes from a pathogen Subsequent signal transduction pathway(s) then carry the signal to a set of downstream target genes that initiate the host defenses Dangl, 2001) The R gene-mediated defenses typically involve a rapid, localized necrosis, or hypersensitive response (HR) at the site of infection and the localized formation of antimicrobial chemicals and proteins that restrict growth of the pathogen (Greenberg, 1997).
Numerous R genes have been cloned and characterized from dicots (Hulbert et al., 2001). However, only several R genes have been cloned from the cereals that contribute heavily to the supply of food for humans and feed for livestock, in part because of the complex genomes of most of these crop plants. In rice (Oryza sativa), the first cloned R gene was the Xa2l bacterial blight R gene that encodes a protein with putative extracellular receptor and cytoplasmic kinase domains (Song et al. 1995; U.S. Pat. No. 5,859,339). Three other rice R genes, the Xal bacterial blight R gene (Yoshmura et al., 1998) and two rice blast R genes, Pi-b (Wang et al., 1999) and Pi-ta (Bryan et al., 2000; U.S. Pat. No. 6,479,731 B1) encode putative cytoplasmic receptor proteins with a nucleotide binding site. Pita protein also shows specifically physical interaction with AVR-Pi-ta protein both in the yeast two-hybrid system and an in vitro binding assay (Jia et al., 2000).
Bacterial blight (BB) disease caused by Xanthomonas species affects virtually all crop plants and leads to extensive crop losses worldwide. BB in rice, caused by Xanthomonas oryzae pv oryzae (Xoo) has been one of the most serious disease in rice, affecting production in irrigated and rain-ed lowland ecosystems throughout Asia, northern Australia, mainland Africa, the southern part of United States and Latin America. Yield loss due to the disease ranges from 20 to 30% (Ou 1985). To date, the best control for BB in rice has been the use of varietal resistance. More than 20 R genes against BB have been identified (Kinoshita 1995; Lin et al., 1996; Zhang et al., 1998; Khush et al., 1999; Gao et al., 2001) and two, Xa2l and Xal, have been cloned. Xa2l originates from the wild species Oryza longistaminata and confers resistance to multiple Xoo isolates in transgenic plants (Wang et al., 1996) while Xal confers a high level of race-specific resistance to race 1 strains of Xoo in Japan (Yoshmura et al., 1998).
A novel BB-resistant locus was identified from a BC2 plant (plant 78-1) of a cross between cultivated rice O. sativa cv IR3191745-3-2 (I3 1917) and wild rice species Oryza minuta Acc. 101141 (Amante-Bordeos et al., 1992). This report also suggested that a single gene or closely linked genes conferred race-specific resistance to Xoo stain PXO99 (race 6) of the BB pathogen from the Philippines.